A designer must compare two small commercial sites: a sunny desert roof with 5.8 peak sun hours and a cloudy coastal roof with 3.2 peak sun hours using the same 10-kW array. Which application of peak sun hours should the designer use to predict the difference in daily performance?

The correct method estimates daily energy by multiplying the array’s 10-kW rating by each site’s peak sun hours, then applying appropriate derates for real-world losses. Before derating, this gives 58 kWh/day for the desert roof and 32 kWh/day for the coastal roof; actual yields will be lower, and the ratio indicates the expected difference under comparable system conditions. Using annual plane-of-array irradiation with a PV model is a sound way to estimate yearly production, but it answers a different time-scale question and can conceal daily variation. Multiplying array capacity by peak sun hours while omitting temperature and balance-of-system losses applies the right basic relationship but overstates delivered energy, so it fails the requirement to account for derates. Converting peak sun hours into equivalent full-sun hours is useful in battery-autonomy planning, but nighttime runtime depends on battery capacity, usable depth of discharge, and load; that calculation does not predict the array’s daily energy yield. Exam caveat: peak sun hours are equivalent full-irradiance hours, not the number of clock hours between sunrise and sunset. In practice, verify that the resource data use a consistent orientation and time period, and use site-specific loss assumptions or a validated PV model for the final estimate.